In a typical radio access network (RAN), an area is divided geographically into a number of wireless coverage areas, such as cells and sectors, each defined by a radio frequency (RF) radiation pattern from a respective base transceiver station (BTS) antenna. The base station antennae in the cells are in turn coupled to a radio network controller (RNC), which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (MSC) or packet data serving node (PDSN) for instance. The switch or gateway may then be coupled with a transport network, such as the PSTN or a packet-switched network (e.g., the Internet). An access terminal communicates with a BTS via an RF air interface. A communication path is thus established between the access terminal and the RAN.
One or more sectors may be defined by radiation patterns from directional-antenna components of a respective BTS, or by respective BTS antennae. These sectors (which can be visualized ideally as pie pieces) can be referred to as “physical sectors,” since they are physical areas of a cell site. Therefore, at any given instance, an access terminal in a RAN will typically be positioned in a given physical sector and will be able to communicate with the transport network via the BTS serving that physical sector.
As an access terminal moves between coverage areas of the RAN, as network conditions change, or for any other reason, the access terminal may switch from receiving service in one coverage area to receiving service in another coverage area. In a usual case, this switching process is triggered by the access terminal monitoring the signal conditions in various available coverage areas of the RAN, and the access terminal and/or the RAN determining when one or more threshold criteria are met.
In one typical example, the access terminal may monitor signal strengths in various available sectors operating on the same air-interface protocol and then decide to switch to a new sector when the new sector has a signal strength that is sufficiently higher than the signal strength of the sector in which the access terminal is currently operating. The access terminal may then notify the RAN of its decision to switch sectors. In turn, the RAN may act upon the access terminal's request to switch sectors by ending service to the access terminal in the current sector and beginning service to the access terminal in the new sector. Handoff between sectors operating on the same air-interface protocol is defined as a horizontal handoff.
Additionally, an access terminal may determine that a new coverage area is capable of communicating with the access terminal on a different air-interface protocol, and may then request to switch to the different air-interface protocol, which may be offered by the same base transceiver station as the current air-interface protocol or by a different base transceiver station. For example, an access terminal may request handoff from an IS-2000 (e.g., 1xRTT) air-interface protocol offered by a first nearby base transceiver station to an IS-856 (EV-DO) air-interface protocol offered by a same or different base transceiver station. The coverage areas defined by the IS-2000 air-interface protocol and the IS-856 air-interface protocol may fully overlap, partially overlap, or not overlap at all. Handoff between coverage areas operating on different air-interface protocols is defined as a vertical handoff.